Galaxy S4: Unlocking when on a call?

So, I can't seem to find an answer to this. Most posts seem to say it is the proximity sensor, but I don't think that is it. Anyway......

I have a Galaxy S4. I use the pattern screen lock. If the phone is locked when I receive an incoming call, I can answer it by swiping. However, the phone remains locked. I have no way to switch it to speaker phone, look up a contact, or even hang up. The solution for hanging up calls is to set the power button on the side to hang up calls, but that doesn't really solve the overall problem.

The reason I don't think this is the proximity sensor is because if I have the phone unlocked when placing or receiving calls, it blanks out and turns back on properly. Also, when receiving an incoming call when the phone is locked, I don't even have to put it up to my ear for the screen to blank. I can just leave it on the table/desk.

Thank you for the reply and thats understood. When receiving a call when the phone is locked & after answering, if you press the home button before the screen auto locks, it takes you to the lock screen. But if the screen locks during the call and you press the home button you should be prompted to the lock screen. So whats the difference?!! I dont understand samsungs brain!!!